Four Pan-African plutonic sets of the Colomines gold district (East-Cameroon): Petrogenesis, K-Ar dating and geodynamic significance

Abstract

Abstract The study of Colomines area (eastern Cameroon) is designed to furnish petrological, mineralogical, geochemical and K-Ar data on granitoids in order to discuss the thermodynamic conditions of crystallization of different magmas , the magma series and differentiation processes, the characteristics of magmas’ sources and the geodynamic implications. The Colomines area is made up of four intrusive sets (heterogeneous granodiorites and their mafic enclaves, mafic plutonic suite (MPS), porphyritic biotite granites , Ms-Bt granites). Mineralogical assemblage is constant in MPS and mafic enclaves (Cpx+Hbl+Bt+Pl+Qtz+Op+Sph+Zrn+Ap), and granodiorite and granites sets (Hbl+Bt+Ms+Pl+Kfs+Op+Sph+Zrn+Ap), except the leucocratic Ms-Bt granites which are amphibole-free. Amphibole in diorite (MPS) and heterogeneous granodiorite has a composition of edenite and ferroedenite, respectively, whereas biotite composition varies with an increase of Fe content from basic to acidic rocks. Concomitantly, plagioclase show an increase of albite component. Crystallization of magmas started at great depths >22\xa0km but the coexistence of heterogeneous granodiorites, MPS plutonic suite rocks, porphyritic biotite-granites and Ms-Bt granites at the same crustal level indicates that magmas migrated upwards and emplaced at a depth upper crust . Chemical analyses show that all rocks are magnesian except ferroan Ms-Bt granites and a few samples of microgranites and porphyritic granites. Granodiorites, few monzonites of MPS and all granites are peraluminous whereas mafic enclaves in heterogeneous granodiorites are metaluminous. Heterogeneous granodiorites, gabbros and diorites of the MPS are calcic and calc-alkalic while other rock types are alkali-calcic and alkalic. Porphyritic granites and Ms-Bt granites trend from calc-alkaline to shoshonitic series. It is suggested that a fractional crystallization process may have contributed to the transition from blind primary magmas to these evolved granitoids. However, each group of rocks does not correspond to a magmatic sequence resulting from a single parent magma by fractional crystallization. They are rather indicative of the heterogeneity of magmatic sources and complex melting processes. The granitoids and associated rocks of Colomines correspond to complex mixtures in varying proportions of magmas resulting from the melting of the subducted crust and associated sediments, the mantle wedge and the lower crust . Biotite and rock compositions typical of volcanic-arc and syn-collision-related granites, and the compositions of Ms-Bt granites typical of post-collisional granites point to an active continental margin setting. The Colomines area has recorded most of the main magmatic events of the Pan-African orogeny from ante-collisional D1 (e.g.\xa0>\xa0640\xa0Ma heterogeneous granodiorites of Kaleka hill), syn-collisional D2 (620-600\xa0Ma migmatisation and anatexis : leucosomes of Kaleka hill), late collisional D3 (590-580\xa0Ma MPS) to Post-collisional D4 (580-560\xa0Ma porphyritic biotite granites and Mu-Bt granites) intrusions.